One aspect of the present invention relates to an improved visible lamp, and particularly to such a lamp which has a controllable spectral output.
As is well known, the color of the light which is provided by a lamp is determined by the spectral energy distribution of the emitted radiation. In general, visible light sources emit over the spectral range of 350-750 nanometers.
It is desirable to be able to control the "tint" of a lamp which nominally emits over the entire visible spectrum. For example, for certain applications it may be desirable for the light to be tinted red, while for certain other applications, a green tint may be preferred. In addition to being able to provide different lamps having different tints, it is also desirable to be able to vary the tint or spectral emphasis of the light which is emitted by a particular lamp during operation.
In the prior art, discharge lamps are typically provided with different spectral emphases by employing fill additives. For example, a metal halide lamp which is doped with thallium emphasizes the green part of the spectrum, whereas one which is doped with sodium would emphasize the yellow. One disadvantage of such lamps is that a different additive or combination of additives must be used to make each differently tinted lamp, thus introducing manufacturing complexities. Additionally, due to the fact that different fill substances have different aging characteristics, the spectra of lamps using additives are prone to change over time.
Another approach to modifying the color output of a lamp is to use external filters. However, such devices inevitably reduce the efficacy of the overall lamp system. It is also known that incandescent lamps can be made more red by reducing the operating temperature of the filament, but this also has the effect of reducing the lamp efficacy.
Additionally, the above schemes which are known to the prior art change the color emphasis of the light output by changing the shape of the overall spectral distribution, i.e., by emphasizing one portion of the spectrum but not others. However, it has been found that for certain applications, it is advantageous to change the color emphasis while retaining substantially the same shape for the overall spectral distribution. For example, in red/green/blue (RGB) color reproduction systems such as a liquid crystal display (LCD) high definition television, it is desirable to provide a lamp having a spectral energy distribution which can emphasize the blue or red without substantially distorting the shape of the overall distribution.
In U.S. applications Ser. No. 779,718, filed Oct. 23, 1991, Ser. No. 604,487, filed Oct. 25, 1990, Ser. No. 882,410, filed May 13, 1992 and Ser. No. 08/071,027, filed Jun. 3, 1993, now Pat. No. 5,404,076, all of which are incorporated herein by reference, a new type of discharge lamp is disclosed which uses a fill which contains a sulfur or selenium containing substance. The fill is present at a pressure of at least about 1 atmosphere, and is excited at a relatively high power density. The lamp produces a molecular spectrum in the visible part of the spectrum at a relatively high efficacy and has exhibited a long lifetime and a stable color output over time.
While the lamp disclosed in the prior applications has many advantageous properties, when not used in accordance with an aspect of the present invention, the spectral output or color temperature may vary around the periphery of the bulb. It is of course desirable for many applications, for the spectral output to be uniform around the bulb surface, so that all portions of the illuminating energy appear to be the same color.
It has also been found that the above-described spatial "color separation" effect may become more pronounced when the discharge lamp is operated at low power levels. Furthermore, at such power levels the bulb may extinguish or the discharge may retreat from the bulb walls.